Metal production in M 33: space and time variations

Context. Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution. Aims. We analyse the spatial distribution of metals in M 33 using a new sample and the literature data on H II regions, and constrain a model of galactic chemical evolution with H II region and planetary nebula (PN) abundances. Methods. We consider chemical abundances of a new sample of H II regions complemented with previous data sets. We compared H II region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we followed the time evolution of the baryonic components and chemical abundances in the disk of M 33, assuming that the galaxy is accreting gas from an external reservoir. Results. From the sample of H II regions, we find that i) the 2D metallicity distribution has an off-centre peak located in the southern arm; II) the oxygen abundance gradients in the northern and southern sectors, as well as in the nearest and farthest sides, are identical within the uncertainties, with slopes around -0.03-4 dex kpc(-1); iii) bright giant H II regions have a steeper abundance gradient than the other H II regions; iv) H II regions and PNe have O/H gradients very close within the errors; v) our updated evolutionary model is able to reproduce the new observational constraints, as well as the metallicity gradient and its evolution. Conclusions. Supported by a uniform sample of nebular spectroscopic observations, we conclude that i) the metallicity distribution in M 33 is very complex, showing a central depression in metallicity probably due to observational bias; II) the metallicity gradient in the disk of M 33 has a slope of -0.037 +/- 0.009 dex kpc(-1) in the whole radial range up to similar to 8 kpc, and -0.044 +/- 0.009 dex kpc(-1) excluding the central kpc; iii) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present.